The present invention relates to a loading box adjustable in zones thereof for a former of a paper or board machine, which former comprises a twin-wire section consisting of two wires placed one above the other and running in the same direction. In the twin-wire section, a drain box is mounted on one side of the wires and removes water from a fibrous pulp running between the wires. Also in the twin-wire section, a loading box is mounted at the opposite side of the wires, and preferably in opposed relationship to the drain box, and is provided with a number of loading ribs which are placed in the transverse direction in relation to a transfer direction in which the wires run and which extend across the web width. The loading ribs are loaded toward the bottom of the drain box in order to produce a desired compression in the fibrous pulp running between the wires.
Conventionally, a former of a paper machine comprises a twin-wire section in which an upper wire and a lower wire run substantially parallel and one above the other (in a horizontal former). The fibrous pulp is introduced between the wires for draining. On the top of the upper wire, there is a drain box in which a vacuum is maintained in order to absorb water out of the fibrous pulp. The lower wire is normally supported by means of a loading box which is provided with loading ribs transverse to the running direction of the wires and which is stationary in relation to the drain box. In such a former, it is desirable that the magnitude of the gap between the wires can be changed and that the shape of the gap in relation to the transfer direction of the wires can also be changed. For this purpose, in the prior art, a number of different modes have been described for guiding and supporting the wires.
For example, in DE Patent No. 3,406,217, a wire guide path is described in which the lower wire is supported by means of a number of ribs placed side by side and which extend across the width of the wire. The lower wire rests against the ribs, and the ribs are pressed adjustably against the lower wire. In this construction, the ribs are placed tightly adjacent to one another which results in the drawback that the ribs act upon one another by the intermediate of friction, for which reason it is difficult to provide a precise control of the loading provided by the ribs. From DE Patent No. 3,153,305, a wire guide path is known in which a number of ribs are employed. The ribs are arranged at a distance from one another, and rest and are supported against the lower wire. The pressing of these ribs against the lower wire is adjusted individually by means of spring members.
From Finnish Patent No. 90,572, a construction is known in which the loading ribs are loaded against the lower wire by means of loading hoses arranged parallel to the longitudinal direction of the ribs, i.e., transverse to the running direction of the wires. The desired compression of the ribs against the lower wire is produced by regulating the pressure in the hoses. By means of the construction in accordance with this Finnish patent, each rib can be loaded with a force of the desired magnitude against the lower wire, e.g., such that the loading of the ribs increases in the running direction of the wires.
It has been a substantial drawback of the prior art constructions described above that it has not been possible to make the moisture content of the web in the cross direction, i.e., the direction across the web width, uniform. Rather, in the prior art constructions mentioned above, the moisture curve or moisture profile in the cross direction has become such that the moisture content in the web is considerably higher in the lateral areas of the web than in the middle areas of the web. This results primarily from the fact that the rib is loaded against the lower wire substantially with a uniform load, in which case the rib that "floats" on a loading hose subjected to a uniform pressure behaves such that, owing to the points of discontinuity, at the ends of the rib, a torque is formed which attempts to bend the rib. Thus, by means of the uniform loading of the rib, a uniform compression of the rib against the lower wire is not achieved across the entire web width. In the prior art, attempts have been made to solve this adjustability or profiling in the cross direction, among other things, so that, for example, in a construction in accordance with Finnish Patent No. 90,572, the loading hose placed below each rib has been divided into separate chambers in the longitudinal direction of the rib, and the pressure in each of the chambers is separately adjustable. By means of such a construction, adjustability in zones of the loading of the ribs is achieved, but the actual implementation and realization of this construction is highly complicated and quite difficult to control.